The success of any digitally-based radio frequency ("RF") communications system is dependent upon the structures that are used to create, store, transfer, and modify data used within that system. This is particularly true when the user data may be encrypted. As is known, there are several difficulties with such structures, or frames. First, in order to guarantee that the RF communications link is established and maintained it is important to include as much addressing and control for the RF repeater as possible. Second, an encrypted message requires a separate synchronization data field to allow listeners to re-synchronize their encryption algorithms as often as possible without using excessive channel capacity.
A typical RF system (100) is shown in FIG. 1. There is shown a repeater (10), a first mobile unit (11), and a second mobile unit (12). As shown, the first mobile unit transmits inbound frames to the repeater via an inbound channel (16), and the repeater transmits outbound frames to the second mobile unit via an outbound channel (17). In FIG. 1, it is assumed that the first mobile transmits the inbound frame (18) to the repeater which, after processing, transmits it as the outbound frame (19) to the second mobile.
Referring now at the inbound frame (18), the frame consists of a signalling mark (20), a network access code (22), an inbound link control field (13), an encryption synchronization field (24), and an encrypted voice field (26). Likewise, the outbound frame (19) consists of a signalling mark (21), a network access code (23), an outbound link control field (14), an encryption synchronization field (25), and an encrypted voice field (27).
As is known in the art, the repeater will normally repeat the link control field. In this case, the inbound link control field (13) will be identical to the outbound link control field (14). Optionally, however, the repeater may delete or delay the inbound link control field (13) to insert a priority outbound link control field (14). In this case, the inbound and outbound link control fields will be different. As is known, the repeater never alters the encryption synchronization field.
Note that, since there are two links, an inbound and an outbound, the inbound link control field (13) and the outbound link control field (14) are not necessarily the same.
A typical frame (200), as in the prior art, that may be used in FIG. 1 is shown in FIG. 2. There is shown a frame (200), whose total bit length (261) may be, for instance, 3344 bits. The total frame (200) comprises an encrypted voice and application data (209) of 2704 bits, a 48 bit synchronization mark (101), a 64 bit network access code (103), a link control field (205) comprising 240 bits, and an encryption synchronization field (207) of 288 bits. Hereinafter, the encryption synchronization field (207) is alternately referred to as "esync".
As is known, in order to efficiently provide RF link signalling requires that short address fields be inserted as frequently as possible to allow the RF link to be controlled. This includes endpoint addressing, link establishment, and link disconnect. To efficiently provide crypto sync, comparatively large synchronization fields must be inserted into the data stream in an efficient manner.
A key problem faced by an RF communication system using a prior art frame (200) is to provide synchronization capability for the encryption so that the receivers can synchronize their decryption in the middle of the message. This occurs for radios that are scanning as well as radios that experience a fade at the beginning of the message.
Cryptographic synchronization requires at least 64 bits and up to 94 bits of information depending on the cryptographic system being employed. Performance at error rates up to 10% are desirable. If a rate 1/2 code is used to provide error correction then at least 188 bits of embedded signalling are necessary for synchronization and the signalling is likely to work only up to 5% bit error rate (hereinafter "BER"). The provision of 188 bits of signalling is sufficient to require a large frame structure made up of several frames of VSELP voice data, each VSELP data frame occupying at least 144 bits. For existing radio systems, the esync field (207) provides 288 bits in order to pass at least 94 bits of information.
Those readers who desire further information on VSELP are directed to the following U.S. patents: "Digital Speech Coder Having Improved Vector Excitation Source", Ira A. Gerson, U.S. Pat. No. 4,817,157, issued Mar. 28, 1989, and "Digital Speech Coder Having Improved Vector Excitation Source", Ira A. Gerson, U.S. Pat. No. 4,896,361, issued Jan. 23, 1990. Both patents are assigned to Motorola, Inc.
The foregoing U.S. patents are hereby incorporated by reference.
It is possible to reduce the channel capacity burden for esync by inserting the esync message every 10 to 20 voice frames. In this way, the channel capacity required for esync can be reduced to a fraction of the capacity allocated to voice. This has the effect of spreading out the esync over a time interval ranging from 1/3 second (333 milliseconds) to more than 1 second.
Current systems provide an esync for every 12 voice frames, each frame encoding 30 millisecond of speech. This provides a re-sync every 360 millisecond. If the data link control (205) and addressing information (103) were distributed over the same intervals, then very substantial delays for data link signals would be incurred to the detriment of a total system with many other services besides encrypted voice.
Therefore, it is desirable to provide an improved frame structure.